I don't know if that is a bit of a how long is a piece of string question. I've been having issues with this too. When can you confirm that a tool is reliable and valid? It's a bit like saturation, there's no clear cut answers. I've been testing one question for the past few years with over 600 people and I'm still unsure and dissatisfied! I don't know if the following link might be helpful with your testing: